The present invention relates to a continuous process for the production of nitrobenzene by adiabatic nitration of benzene by a mixture of sulfuric and nitric acid (so-called mixed acid). A process of this type was first claimed in U.S. Pat. No. 2,256,999 and is described in more modern embodiments in U.S. Pat. Nos. 4,091,042, 5,313,009 and 5,763,697.
Common to the adiabatic processes described is the fact that the starting substances benzene and nitric acid are reacted in a large excess of sulfuric acid, which takes up the heat of reaction liberated and the water formed during the reaction.
The reaction route generally involves combining the nitric acid and sulfuric acid to form so-called nitrating acid (also known as mixed acid). Benzene is metered into this nitrating acid. The reaction products are substantially water and nitrobenzene. In the nitration reaction, benzene is used at least in a stoichiometric quantity, based on the molar quantity of nitric acid, but preferably in a 2% to 10% excess. The crude nitrobenzene formed in the reaction apparatus and separated off from the acid phase in the phase separation apparatus is subjected to washing and a work-up by distillation according to the prior art, as described for example in EP 1 816 117 A1 (page 2, lines 26 to 42), U.S. Pat. No. 4,091,042 (see above) or U.S. Pat. No. 5,763,697 (see above). It is characteristic of this work-up that unreacted excess benzene is separated from nitrobenzene in a final distillation after the wash and reused in the nitration reaction as recycled benzene, which also comprises low-boiling, non-aromatic organic compounds (so-called low boilers) (cf. DE 10 2009 005 324 A1). The treatment of the exhaust gas from the adiabatic nitration reaction is described in EP 0 976 718 B1. The exhaust gas of circulating acid and finished crude nitrobenzene is drawn off, combined and sent through an NOx absorber to recover dilute nitric acid, which can be returned into the reaction. The sulfuric acid referred to as circulating acid is concentrated in a flash evaporator and freed from organics as far as possible. High-boiling organics, such as e.g. nitrobenzene, dinitrobenzene and nitrophenols, remain in the circulating acid in traces and are therefore also returned to the reaction.
The quality of an adiabatic process for the nitration of aromatic hydrocarbons is defined on the one hand by the product's content of undesired by-products of the reaction, which are formed by multiple nitration or oxidation of the aromatic hydrocarbon or of the nitroaromatic. The aim in the production of nitrobenzene is to minimise the content of dinitrobenzene and of nitrophenols, in particular of trinitrophenol (picric acid), which is classified as explosive. On the other hand, the quality of an adiabatic process is defined by the ability of the process to be operated without any industrial production losses.
In order to obtain nitrobenzene with particularly high selectivities, the nature of the mixed acid to be used has been specified in detail (EP 0 373 966 B1, EP 0 436 443 B1 and EP 0 771 783 B1), and it has been pointed out that the content of by-products is determined by how high the maximum temperature is (EP 0 436 443 B1, column 15, lines 22 to 25). It is also known that a high initial conversion is advantageous for high selectivity and that this is achieved if optimum mixing is effected at the beginning of the reaction (EP 0 771 783 B1, paragraph [0014]).
Excellent selectivities are achieved if the initial reaction temperature is selected to be very low (WO2010/051616 A1), but this equates to a much longer reaction period. A high space-time yield is advantageous for the industrial application of a process, since this enables compact reaction equipment to be constructed which is distinguished by a low investment volume relative to capacity. This approach is therefore counter-productive.
It is common to all of the literature references cited that they do not describe the start-up process of a nitration plant and its difficulties.
With regard to the quality of the feed substance benzene on the adiabatic production of nitrobenzene, EP 2 246 320 A1 describes that commercially available benzene can be contaminated to a greater or lesser degree depending on its source. Typical impurities are other aromatics, in particular toluene and xylene, which can be comprised in benzene of standard purity in a quantity of up to 0.05 wt. % in each case. Other typical impurities for benzene are non-aromatic organic compounds, which can constitute a total of up to 0.07 wt. %. Cyclohexane (up to 0.03 wt. %) and methylcyclohexane (up to 0.02 wt. %) are mentioned in particular here. The impurities described above in the concentrations mentioned have either no negative effect at all in the subsequent steps in the process chain for the production of di- and polyisocyanates of the diphenylmethane series (MDI) or only a slight one, for example by minimally increasing the difficulty of waste water and exhaust air treatment in the nitrobenzene process as a result of non-aromatic organics in the benzene. Costly purification of benzene for use in the MDI process chain is therefore considered disproportionate and can be omitted. EP 2 246 320 A1 does not go into the non-aromatic organic compounds in the benzene that has been separated off from the crude product after completion of the reaction and returned into the nitration (so-called “recycled benzene”).
DE 10 2009 005 324 A1 discloses that technical benzene generally has a proportion of low-boiling non-aromatic organic compounds (low boilers) of 0.01 to 0.5%. In the common processes of benzene nitration, however, it is not technical benzene as such that is used but a mixture of recycled benzene and technical benzene, so that the content of low boilers in the benzene actually used can be considerably higher than in commercially available technical benzene. DE 10 2009 005 324 A1 discloses a value of 5% by way of example (section [0007]). According to the teaching of this document, such a high proportion of low boilers in the actual nitration is still not disadvantageous. DE 10 2009 005 324 A1 only goes into problems with the subsequent phase separation (section [0008]). To solve these problems, a special phase separation process is proposed, using a pressure-maintaining siphon.
EP 2 155 655 B1 only goes into aromatic impurities (alkyl-substituted aromatics) in benzene.
DE-AS-1 129 466 describes a process for the mononitration of technical benzene, xylene and toluene, which have the conventional contents of aliphatic hydrocarbons, in which the first runnings of unreacted aromatic compound formed in the distillation of the nitroaromatic compound, which are rich in aliphatic impurities, are mixed with fresh aromatic compound, supplied to the nitration and the newly formed first runnings are circulated as often as desired. (In the case of benzene as the starting substance, the aromatic first runnings in this document correspond to the above-mentioned recycled benzene.) The person skilled in the art therefore takes from this document the technical teaching that a relatively high proportion of aliphatic impurities in the aromatic compound to be used has no negative effect on nitration.
It is true that the processes of the prior art described succeed in producing a nitrobenzene having a low content of by-products, i.e. comprising only from about 100 ppm to 300 ppm of dinitrobenzene and 1500 ppm to 2500 ppm of nitrophenols, with picric acid possibly making up a proportion of 10 wt. % to 50 wt. % of the nitrophenols. The processes are also distinguished by a high space-time yield.
However, only processes which are already in progress are described, i.e. in which the period from the beginning of the reaction to achieving the target load (so-called “start-up period”) has already passed. Any particular difficulties during start-up of an adiabatic nitration process are not mentioned.